The present invention relates generally to gamma density logging tools and more particularly to a novel technique for stabilizing such detectors. Still more particularly, the present invention comprises a method for more accurately adjusting the gain of a stabilizer so that the center of the stabilization peak is positioned at a desired energy level.
Gamma density logging is well known in the art of petroleum drilling. In gamma density logging, a source of gamma rays is lowered into the hole, along with at least one, and more typically at least two gamma ray detectors. Each detector typically comprises a large thallium-doped sodium iodide NaI(Tl) crystal coupled to a low-noise photomultiplier. The incidence of gamma rays on the detectors, along with known information about the respective distances of each detector from the GR source, gives information about the effect of the surrounding formation on the gamma rays as they are transmitted from the source to the detectors, and thus gives information about the formation itself.
In nuclear radiation measurement techniques and also in photometry use is made of detectors such as scintillation counters, proportionally counter tubes, semiconductors, or photomultipliers having internal amplification, which supply pulses whose pulse amplitude corresponds to the energy absorbed by the detector. Gamma density logging tools typically use a gamma ray source in conjunction with one or more scintillation counters. Gamma rays from the source are emitted into the formation surrounding the tool. The formation scatters the gamma rays, causing some of them to return to the tool and impact the scintillation counters. The counters convert energy from these incident gamma rays into light, which is in turn detected and amplified by a photomultiplier, which emits a signal corresponding to the energy of the incident gamma rays.
A particular difficulty arises as a result of drift in such radiation measurement means, especially because of variations in internal amplification. These variations are particularly severe as the temperature of the tool rises. These variations gives rise to variations in pulse heights and consequently to a displacement of the energy lines or energy peaks with respect to the adjusted response thresholds of pulse registration, and hence to inaccurate measurements of intensity. The variations can be kept within acceptable limits by regular checking with calibrating radiation sources and adjustment of the tool. However, periodic calibration of the energy spectrum with external sources is not possible when the tool is in use (logging). For this reason, and because the gain changes due to elevated temperatures in a well can be quite large, automatic control methods and devices for stabilization of drift are preferred.
In order to avoid the disadvantages associated with drift, it is desirable to use a peak of the pulse amplitude spectrum as the controlling variable in automatic stabilization techniques. For this to be possible, the pulse amplitude spectrum of the radiation must have a peak that is discernable and suitable for control purposes.
Typical gamma-ray logging tools use a cesium-137 gamma source and two density detectors spaced apart from the source. Cesium-137 emits gamma rays with an energy of 662 keV. The standard means for stabilizing such a detector in a gamma-gamma density tool is to place a small cesium-137 source on or near the surface of each detector. A representative spectrum of the stabilization source energies detected at the far detector is shown in FIG. 1. It can be seen that there is a clear peak at 662 keV, but there are also a large number of counts at lower energy levels, which correspond to smaller amounts of energy that were absorbed in the scintillation crystal.
In operation, the observed spectrum for a tool of this sort comprises the sum of the logging and stabilization spectra. Fortunately, the energy loss in the formation is great enough that the energy of the gamma rays detected after passage through the formation is much lower than their original energy, as shown in FIG. 2. In contrast, the energy of the gamma rays originating at the stabilization source does not pass through the formation and so is virtually unchanged when it is detected. The observed spectrum thus includes a stabilization peak at 662 keV superimposed on a spectrum having a much larger peak at lower energies, as in the spectrum of FIG. 3, in which the portion of the spectrum between 40 and 400 keV is off the scale.
It is presently known to use a two step process for distinguishing the stabilization peak from the rest of the spectrum. The two steps entail (1) adjusting the gain of the system so as to place the stabilization peak at approximately 662 keV and (2) adjusting the gain until the count rate in the energy range from 617 to 662 keV is the same as the count rate in the energy range from 662 to 708 keV. Step (1) is carried out by positioning the peak such that there are enough counts above 617 keV to ensure that the stabilization peak is above that point and so few counts above 708 to ensure that the peak is below that point.
Successful use of this technique depends on three factors. First, there must be relatively few counts above the stabilization peak. Second there must be a well-defined valley between the logging spectrum and the stabilization peak, and third, the stabilization peak must be easily distinguished in the spectrum. In theory, these three conditions can always be met by increasing the strength of the stabilization source. Increasing the strength of the stabilization source also increases the fraction of the spectrum that corresponds to the stabilization source, however. Since the stabilization spectrum extends into the energy region that is used to compute formation density, the portion of the spectrum that is attributable to the stabilization source must be subtracted from the observed spectrum to allow calculation of the formation density. Thus, as the strength of the stabilization source increases, the statistical uncertainty of the desired calculations also increases. This is particularly disadvantageous in situations where the overall spectrum has a low count rate, such as in logging while drilling (LWD) applications and in cased-hole density tools.
Cased-hole density measurements are particularly difficult, since the tool must operate in both very high- and very low-count-rate situations. This is because gamma rays are attenuated exponentially with the density of the rock. High-density formations, such as carbonates, attenuate gamma rays quickly. However, very low density formations, such as coal, attenuate the gamma rays much less. Thus, if a detector far from the source is large enough to detect a reasonable number of counts per second in a dense rock, it will detect many times more counts in a low-density rock. If the tool is used in open holes without casing, the fractional difference between high-density formations behind casing and low-density formations in open holes is even greater, since the casing also causes significant attenuation. Even if the tool is not used in open holes, it is desirable to calibrate it in existing calibration fixtures, which have no casing. Hence, the tool must be able to operate under a wide range of conditions.
Furthermore, if the tool uses a cobalt-60 logging source, which emits gamma rays of 1.17 and 1.33 MeV, the high-count-rate situations have a significant number of counts at energies in the region above 662 keV. FIG. 4, which is an example of an observed spectrum for the third detector of a cased-hole density tool placed in a magnesium block, illustrates the nature of this problem. As can be seen, there are a significant number of counts above 662 keV and there is no well-defined valley between the stabilization peak and the logging spectrum. In situations like this, the conventional two-step technique described above for adjusting gain will not work. In addition, because the stabilization peak is obscured by a large, nonlinear background, the technique of balancing the count rates immediately above and below 662 keV will result in a miscalibrated instrument.
Hence, it is desired to provide a stabilization technique that is effective in situations where there are a significant number of counts above 662 keV, where there is no well-defined valley between the stabilization peak and the logging spectrum, and/or where the stabilization peak is obscured by a large, nonlinear background. The stabilization technique should require sufficiently few calculations as to be operable in the logging context and should be consistently accurate in a variety of logging situations.